This invention relates to back spotfacing tools, and more particularly to having a replaceable cutter, which permits the formation of a larger spotface diameter for a given pilot hole size.
In my U.S. Pat. No. 3,814,535, I disclosed a back spotfacing and counterboring tool of the type in which the spotfacing cutter is pivotal into and out of a recess in the tool shank between a retracted or inoperative position within the shank, and an extended position in which it is engagable with the work which is to be spotfaced. The advantage of such construction is that the cutter can be swung to its retracted position, and the tool with the retracted cutter can then be inserted through an opening or a pilot hole on a workpiece. As soon as the end of the tool bearing the cutter has passed through the workpiece, the cutter can be swung to its open or operative position, after which the rotating shank of the tool can be retracted operatively to engage the cutter with the surface of the workpiece which is to be back spotfaced. While such prior tool is very effective, the diameter of the spotface which can be produced by the tool is limited by the size of the cutter which can be retractably mounted in the tool.
To enable the production of larger spotface or counterbore diameters, certain prior art devices have utilized replaceable back spotfacing cutters, which are designed to be attached to the shank of a tool after the tool has been passed through an opening in the workpiece. The cutter is then attached to the tool shank, and the tool is then retracted to engage the cutter with the surface which is to be back spotfaced. After the spotfacing operation the tool is advanced and the cutter is removed to permit withdrawal of the tool out of the work.
One such prior art device employs a so-called bayonet-lock type of cutter having therethrough an axial bore for accommodating the shank of the tool. A pin which is fixed in the annular wall of the cutter to extend at its inner end part way into cutter bore, is engagable in a generally dogleg-shaped recess formed in the periphery of the tool shank, so that when the shank is inserted into the bore in the cutter and rotated, the projection in the bore becomes releasably seated in the inner end of the recess in the tool, thereby removably attaching the cutter to the tool. The tool can then be retracted to engage the cutter with a surface of the work which is to be back spotfaced. After a spotfacing operation has been completed, the tool is advanced and the cutter can be removed simply by rotating it slightly relative to the tool in a direction to disengage its projection from the inner end of the recess in the tool shank, thereby permitting removal of the cutter from the tool. Among the disadvantages of this bayonet-lock type of cutter is the fact that the associated tool must be operated at a relatively low cutting speed, therefore resulting in low productivity. Also the cutter requires frequent re-sharpening, and such cutter is not designed for forming interrupted or deep cuts in a workpiece.
Still another type of back spotfacer which employs a removable cutter is known as the block type. With this back spotfacer, the end of the tool shank that is to be inserted through an opening in the workpiece has therethrough a diametral slot, which is generally rectangular in configuration. After the slotted end of the tool has been inserted through the opening in a workpiece, a rectangular cutter, which is longer than the diameter of the tool shank, is inserted into the slot so that opposite ends of the cutter extend or project beyond diametrally opposite sides of the tool shank. A set screw is threaded into the shank to have its inner end engage in a recess in the cutter intermediate the ends thereof, thereby to fix the cutter against movement in the tool shank. The projecting ends of the cutter have cutting edges thereon which are engagable with the workpiece when the tool is retracted and rotated to form the back spotfacing in the workpiece. After the operation has been completed, the tool is advanced, the set screw is backed off, and the cutter is removed from the slot in the tool shank, thereby permitting the shank to be withdrawn from the workpiece.
This block-type of back spotfacer has the disadvantage that it also must be operated at low cutting speeds, and must be frequently resharpened. Moreover, a substantial amount of time is lost because of the need for employing a separate tool (a screwdriver or the like) for inserting and removing the cutter from the associated tool shank each time a workpiece is to be back spotfaced.
It is an object of this invention, therefore, to provide an improved back spotfacer of the type described which utilizes a replaceable cutter, which can be quickly and easily attached to, and removed from, the shank of a back spotfacing tool without requiring the use of any special tool or instrument for securing the cutter or releasing it from the tool shank.
Still another object of this invention is to provide an improved back spotfacer or counterboring tool having a readily removable and replaceable cutter, which is capable of being operated at high cutting speeds, and which is particularly suitable when the diameter of the desired back spotface exceeds a predetermined value.
A more specific object of this invention is to provide an improved back spotfacer tool of the type described having a removable cutter blade that is removably mounted on the associated tool shank in such manner that the inertia of the rotating tool tends to retain the blade securely on the tool, and in an operative position relative to the work which is to be spotfaced.
Another object of this invention also is to supply an improved tool of the type described in which the associated cutter may be designed for both front and back spotfacing.
Other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.